The present invention relates to a method and apparatus for active vision image enhancement. More particularly, the invention is directed to a double spectral image sensing method and apparatus for active vision robot welding and surface inspection.
Visual inspection systems are required for quality control and process monitoring in a wide variety of industrial applications. Increasing demand is developping for noncontact, nondestructive, nonperturbating, fast, reliable and moderate-cost inspection systems for use in harsh environmental conditions, such as in the presence of external light noise perturbations, high temperature, smoke, dust and water vapor. For many applications, active vision offers significant advantages over other techniques since it facilitates adaptive behavior using relatively simple image analysis.
The ability of any active vision system to reject foreign light source perturbation is crucial and is a definite cause of problem for applications such as in robot arc welding where the arc light is very intense and constitutes a strong perturbation for active vision of seam welding. Many systems are known which are capable of extirpating useful signals in such harsh conditions. These systems are generally based on the projection of a structured light beam on the joint to be welded, using laser diodes or the like. When viewed at an angle, the projected light takes the shape of the seam and thus provides an image of the seam profile. Simple algorithma are then used to extract the information needed to synchronize the robot movements and the displacements of the torch from these images. Real time operation is mandatory and the simpler the algorithms are, the lesser computer power will be required, resulting in lower cost and more efficient systems. It is very important to submit high quality, noise-free images to the image processor to avoid slow and costly computer discriminating techniques.
Currently available active vision systems generally use a video camera and an optical filter which is placed in front of the camera and which has a central wavelength corresponding to the wavelength of the monochromatic light projected on the surface to be inspected. Because foreign light noise is generally spread over a broad spectrum, it is possible to reject most of the foreign light noise by selecting an optical filter having a narrow band to thereby increase the visibility of the projected light over the background light noise and to thus facilitate image processing. But for light noise emitters such as a welding torch, there is still enough light noise intensity within the band of the optical filter to disturb considerably the quality of the image.
Narrow band optical filters, however, have a limited efficiency in rejecting foreign light noise since it is not practically possible to reduce the bandwidth of such filters under about 2-10 nanometers depending on the optical design. A narrow band optical filter such as in interference filter is temperature dependent and thus the reduction of its bandwidth will require greater temperature stability. Moreover, the effective bandwidth and central wavelength of an interference filter is dependent upon the angle of incidence of the light. The effect becomes increasingly disturbing as the bandwidth of the filter is reduced.
On the other hand, the use of a powerfull projected light source such as a powerfull laser will contribute to increasing the predominance of the desired signal over noise and thus improving the quality of the image. However, the intensity of the projected light will rapidly become unsafe for eyes when used in an area where human operators are standing by. Also, the cost of such a powerfull laser will be prohibitive.